Documentation: usb: add chipidea.txt for how to demo usb OTG HNP and SRP

This patch adds a file chipidea.txt for how to demo chipidea usb OTG HNP and SRP
functions via sysfs input files, any other possible information should be
documented for chipidea usb driver in future can be added into this file.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Li Jun <b47624@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/chipidea.txt

+1. How to test OTG FSM(HNP and SRP)
+-----------------------------------
+To show how to demo OTG HNP and SRP functions via sys input files
+with 2 Freescale i.MX6Q sabre SD boards.
+
+1.1 How to enable OTG FSM in menuconfig
+---------------------------------------
+Select CONFIG_USB_OTG_FSM.
+If you want to check some internal variables for otg fsm,
+select CONFIG_USB_CHIPIDEA_DEBUG, there are 2 files which
+can show otg fsm variables and some controller registers value:
+cat /sys/kernel/debug/ci_hdrc.0/otg
+cat /sys/kernel/debug/ci_hdrc.0/registers
+
+1.2 Test operations
+-------------------
+1) Power up 2 Freescale i.MX6Q sabre SD boards with gadget class driver loaded
+   (e.g. g_mass_storage).
+
+2) Connect 2 boards with usb cable with one end is micro A plug, the other end
+   is micro B plug.
+
+   The A-device(with micro A plug inserted) should enumrate B-device.
+
+3) Role switch
+   On B-device:
+   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req
+
+   if HNP polling is not supported, also need:
+   On A-device:
+   echo 0 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_req
+
+   B-device should take host role and enumrate A-device.
+
+4) A-device switch back to host.
+   On B-device:
+   echo 0 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req
+
+   A-device should switch back to host and enumrate B-device.
+
+5) Remove B-device(unplug micro B plug) and insert again in 10 seconds,
+   A-device should enumrate B-device again.
+
+6) Remove B-device(unplug micro B plug) and insert again after 10 seconds,
+   A-device should NOT enumrate B-device.
+
+   if A-device wants to use bus:
+   On A-device:
+   echo 0 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_drop
+   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_req
+
+   if B-device wants to use bus:
+   On B-device:
+   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req
+
+7) A-device power down the bus.
+   On A-device:
+   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_drop
+
+   A-device should disconnect with B-device and power down the bus.
+
+8) B-device does data pulse for SRP.
+   On B-device:
+   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req
+
+   A-device should resume usb bus and enumrate B-device.
+
+1.3 Reference document
+----------------------
+"On-The-Go and Embedded Host Supplement to the USB Revision 2.0 Specification
+July 27, 2012 Revision 2.0 version 1.1a"